kwired
Electron manager
- Location
- NE Nebraska
Yes there seems to be fair amount of confusion over this 25 ohm requirement.I realize you understand this requirement, but since there is confusion on this, I will explain for others. Its not section 250.56, but its 250.53 (2), Exception. We often hear the NEC requires a minimum of 25 ohms for ground resistance, what the NEC says is if you have a single electrode, it has to be supplemented except if the single electrode measures 25 ohms or less, a supplemental electrode is not required.
When adding electrodes in parallel a second electrode decreases the resistance by 60%, a third by 40, it the law of diminishing returns. In the trade we refer to this as "drive two and go home".
First it only applies to rod, pipe and plate type electrodes. If not using any of those there is no minimum resistance requirement and only other supplemental electrode requirements is if you have a water pipe electrode but no other electrodes.
Second if a single rod, pipe or plate has more than 25 ohms resistance then you must supplement it with another electrode. You don't even have to measure resistance after adding that second electrode, NEC just wants a supplemental electrode if the first one is too high resistance.
Third if you have a specification to achieve a certain level of resistance in your GES then that is above and beyond NEC requirements. Any questions in this area need to be addressed by whoever specified the resistance that must be met, NEC was satisfied when the second rod, pipe or plate was installed.